Demand for secondary batteries as an energy source has been rapidly increased as technology development and demand with respect to mobile devices have increased, and as a result, a significant amount of research into batteries that can meet various demands has been conducted.
Typically, in terms of the shape of the battery, demand for prismatic type batteries and pouch type batteries having a thin profile which may be used in products, such as mobile phones, is high, and, in terms of materials, demand for lithium secondary batteries, such as lithium cobalt polymer batteries having excellent energy density, discharge voltage, and safety, is high.
One of major research projects of these secondary batteries is the improvement of safety. A leading cause of safety-related, incidents in batteries is attributed to the attainment of an abnormal high-temperature state due to a short circuit between a cathode and an anode. That is, under normal circumstances, a separator is disposed between the cathode and the anode to maintain electrical insulation. However, there may be limitations in using a typical separator alone under abnormal misuse or abuse conditions, for example, the occurrence of overcharge or overdischarge of the battery, or dendritic growth of an electrode material or the occurrence of an internal short circuit due to foreign. matter, or the penetration of the battery by sharp objects such as nails and screws, or excessive deformation of the battery by external force.
In general, a microporous membrane formed of a polyolefin resin is mainly used as a separator. However, since the heat resistance temperature thereof is in a range of about 120° C. to about 160° C., heat resistance properties may be insufficient. Therefore, when the internal short circuit occurs, a state of thermal runaway may be attained in which the separator shrinks due to short circuit reaction heat to enlarge a short-circuit part and generate much more reaction heat.
Also, as illustrated in FIG. 1, a secondary battery is generally prepared in a prismatic shape by cutting a cathode and an anode in an appropriate size and stacking several sheets. In this case, since edges of the cathode or the anode that is coated with a polymer electrolyte may include sharp parts having an inconspicuous tiny needle shape, a microscopic internal short circuit may occur in these parts when the electrodes are stacked. Thus, the performance of the battery may be adversely affected. In particular, since the edges have more irregular surfaces than the inside when coating the polymer electrolyte, the edges are not evenly coated. Thus, a short circuit is likely to occur. Also, when upper and lower layers of The electrodes are even slightly misaligned with each other while stacking the electrodes, a short circuit between the cathode and the anode may occur.
Thus, various methods for reducing the possibility of cell deformation, external impact, or a physical short circuit between the cathode and the anode have been studied.
For example, in order to prevent the occurrence of a short circuit caused by moving an electrode assembly in a state in which the battery is completed to allow an electrode tab to be in contact with an upper end of the electrode assembly, there is a method of attaching a predetermined sized insulation tape on the electrode tab near an upper end of a current collector. Typically, a polyimide film is used as the insulation tape, and it is generally recommend that the insulation tape is wrapped to a slightly extended length from the upper end of the current collector to the bottom. Also, in order to prevent loosening, the insulation tape is typically wrapped twice or three times.
However, such a winding operation of the insulation tape may be highly complicated, and in the case that the insulation tape is wrapped to a slightly extended length from the upper end of the current collector to the bottom, the above wrapped portion may cause an increase in the thickness of the electrode assembly. Furthermore, the wrapped portion may be easily loosened during the bending of the electrode tab.